Composite materials reinforced by carbon fibers have been widely utilized for applications in general industry and in the field of sports, e.g. as structural materials of airplanes, vehicles and the like and for tennis rackets, golf shafts, fishing rods and so on, by making use of their high specific strength and specific elasticity. As to the forms of carbon fibers used in those applications, there are e.g. textile fabrics made from continuous fibers, UD sheets in which fibers are aligned in one direction, random sheets made by using cut fibers and nonwoven fabric.
In recent years, composites in which a thermoplastic resin is used as a matrix instead of a traditional thermosetting resin have received attention. However, most of them have been made into a shaped product by injection molding (e.g. Patent Document 1), and therefore reduction in fiber length has occurred during melt-kneading, resulting in lowering of mechanical strength.
In addition, another molding method has been developed in which a base material for molding use, specifically a mat made from discontinuous long fibers and impregnated with a thermoplastic resin, is heated up to a temperature higher than the melting temperature of the thermoplastic resin, charged into a mold adjusted to a temperature lower than the melting temperature or the glass transition temperature, and then formed by mold clamping.
In fiber-reinforced composite materials, as described in Patent Document 2, shaped articles obtained by molding a mat-shaped base material in which discontinuous long fibers are dispersed in X and Y directions (a planar direction) like nonwoven fabric are well-known to be superior in mechanical strength.
Patent Document 3 describes hybrid carbon fiber-reinforced thermoplastic resin composite materials wherein carbon microfibers, such as carbon nanotubes or carbon nanofibers, are added to carbon fibers having lengths of 20 mm or longer and provides prepregs having high bending strength and high strength in the direction lateral to the fiber axis. Reasons for such high strengths are considered to consist in that, when bending stress is imposed, deformation on the compressed side is suppressed because carbon microfibers enhance the compressive elasticity modulus of the matrix-forming thermoplastic resin and buckling of carbon fibers is reduced, and thereby a compression fracture mode is inhibited and thereby high tensile strength of carbon fibers is made effective, and besides, carbon microfibers are low in degree of orientation in the axial direction of carbon fibers and have a reinforcing effect in the direction lateral to the axial direction where reinforcing effect of carbon fibers cannot be expected.
Further, Patent Documents 4 and 5 describes fiber-reinforced materials in which a thermoplastic resin and two types of reinforcing fibers different in length are used.